Method and Apparatus for Acoustic Sensing Using Multiple Optical Pulses

ABSTRACT

An improved technique for acoustic sensing involves, in one embodiment, launching into a medium, a plurality of groups of pulse-modulated electromagnetic-waves. The frequency of electromagnetic waves in a pulse within a group differs from the frequency of the electromagnetic waves in another pulse within the group. The energy scattered by the medium is detected and, in one embodiment, the beat signal may be used to determine a characteristic of the environment of the medium. For example, if the medium is a buried optical fiber into which light pulses have been launched in accordance with the invention, the presence of acoustic waves within the region of the buried fiber can be detected

BACKGROUND OF THE INVENTION

The present invention relates generally to sensing signals usingmultiple pulses of electromagnetic radiation. In a specific embodiment,the invention relates more particularly to acoustic sensing usingmultiple optical pulses.

The telecommunications industry now uses optical fiber optic cables toform the vast majority of its network backbone. With advances intechnology, a single cable bundle can carry many thousands or evenmillions of telephone conversations. With more recent demands forincreased bandwidth for data and Internet traffic, the lack ofredundancy within these networks has become a cause for concern. If alink within a network fails, there may be a significant cost to thenetwork operator in both customer dissatisfaction and lost revenue. Suchfailures may occur, for example, when excavations associated withconstruction sever an optical fiber cable. Accordingly, the protectionof these buried resources is a high priority for network operators.Practices have evolved to protect the fiber when work is scheduled inthe vicinity of the fiber. However, unforeseen network failures stilloccur due to physical damage to the fiber plant.

A buried fiber alarm system that is able to detect and characterizeacoustic signatures along the length of a fiber route would serve as athreat warning system for network operators. This would allow operatorsof the network to take action before critical failure, possibly avoidingdamage to the cable entirely. It would also allow network traffic to bererouted before service was lost. A sensor of this nature would alsofind application in many wide-ranging fields, such as structuralmonitoring and the protection of other vulnerable services such as oiland gas transmission pipelines. Furthermore, such a technology would beideally suited to improvements in perimeter security. A security sensorof this nature would be unobtrusive and, if buried around the perimeterof a sensitive facility, would be virtually impossible to locate anddisable. Certain optical sensor configurations would even remainoperational if the fiber that is part of the sensor configuration wereto be cut around the perimeter, allowing not only the detection of acut, but also enabling determination of the location of the cut.

Much work has been done in the field of fiber optic based acousticsensors. Perhaps the most sensitive techniques involve interferometricsensors. However, determining the location of the disturbance, andisolating a section of fiber from a persistent, non-threatening,disturbance, proves difficult due to the nature of these devices.Limited success has however been achieved using loop architectures, butdue to the reciprocal nature of the loop configuration multipledisturbances of an unknown nature prove virtually impossible to separateand locate independently. This problem extends to anyforward-propagating differential time-delay method.

Perhaps the most useful, truly-distributed, sensing technique employedin the field of fiber sensors is that of optical time-domainreflectometry (OTDR), a schematic representation of which is shown inFIG. 1. A number of varied methods and applications have been disclosedin the literature but the basic distributed scheme involves a shortpulse of light, typically 10-1000 ns in duration, which is launched intoa fiber, usually a single mode fiber. In FIG. 1, the pulse is launchedfrom source 101, through isolator, 102, switch 103 and coupler 104. Asthe light propagates along the fiber under test (FUT), 108, a smallfraction will be scattered by the tiny random fluctuations in therefractive index of the glass (scatter sites). Some of this scatteredlight is captured by the fiber and guided back toward the launch end ofthe fiber. This backscattered light, and its intensity as a function oftime and hence distance along the fiber, can then be directly detectedby, for example, a PIN diode detector, 105, through coupler 104. Thebackscattered signal may be recovered and displayed on oscilloscope 106.In the FIG. 107 is a method or device to prevent unwanted reflectionfrom the unused port of the 3 dB coupler 104. FIG. 2 is a schematicrepresentation of the backscattered signal showing a discontinuity, 201,associated with optical loss at a splice.

Other detection methods have been disclosed such as optical heterodyne,homodyne and optical amplification methods such as SOA (Solid-stateoptical amplification) and EDFA (Erbium doped fiber amplification).Unlike optical amplification techniques, optical heterodyne and homodynemethods require a coherent source and hence are termed coherent-OTDR orC-OTDR. Typically, the basic OTDR technique has found application in themeasurement and characterization of waveguide features, such as theattenuation of the fiber, splice positions and loss, measurement ofreflective markers and the certification of telecommunicationinstallations. A representative C-ODTR technique is shown in FIG. 3. Inthis FIG, like items from FIG. 1 are labeled as in FIG. 1. The majordifference in this technique is that a sample of the launch light andthe backscattered light combine at the 3 dB coupler, 104, just beforethe detectors, 105, resulting in interference at the detectors, 105. Adifferential detector 109 may then be used to recover the backscatterintensity for exemplary analysis at oscilloscope 106.

In the mid 1980s it was noted that the use of a coherent excitationpulse (i.e. when the coherence length of the source is much greater thanthe pulse length T_(c)>L) in OTDR raised some interesting issues(Healey, P. “Fading in Heterodyne ODTR”, Electronic Letters, 20(1), pg30, (1984)). It was noted that in the exemplary arrangement of FIG. 3the backscattered trace is no longer a predictable, logarithmicallyfalling signal, due to fiber loss, but that this predicted trace ismodulated by a random variable. This random variable is due in part tothe fact that the intensity of the light arriving on the detector at anyspecific time is the coherent addition of the light scattered from manydiscrete scatter sites. This “fading” mechanism is comparable to laserspeckle, a random interference pattern caused by the interference oflight scattering from different positions over the area of a spatiallycoherent beam. It was also noted that due to mechanical (such asvibration) and temperature changes, this random pattern is altered frompulse to pulse as the distribution of the scatter sites at a givenlocation is also altered. This phenomenon was not exploited for sensinga change in the variables associated with the environment in which thefiber is located.

In U.S. Pat. No. 5,194,847 the same phenomenon is described, and issuggested for use in sensing of strain disturbances along the length ofa standard single mode fiber, specifically for the detection ofintrusion across a perimeter. In that patent there is described a systemthat generates a coherent pulse of light from a coherent source. Thispulse is then directly launched along the fiber under test. Thebackscattered radiation is then detected by a square-law detectionsystem, allowing the intensity of the backscattered signal to beobserved. By detecting the change in this intensity for a given fibersection, information about the acoustic signal acting on the fiber canbe recovered.

In U.S. Patent Publication 20050196174, “a method and apparatus isprovided for obtaining status information from a given location along anoptical transmission path. The method begins by generating a continuouswave (cw) probe signal having a prescribed frequency that is swept overa prescribed frequency range. The cw probe signal is transmitted overthe optical path and a returned C-OTDR signal, in which statusinformation concerning the optical path is embodied, is received overthe optical path. A receiving frequency within the prescribed frequencyrange of the returned C-OTDR signal is detected to obtain the statusinformation. The detecting step includes the step of sweeping thereceiving frequency at a rate equal to that of the prescribed frequency.A period associated with the receiving frequency is temporally offsetfrom a period associated with the prescribed frequency.” The fadingproblem discussed above remains in the disclosed method.

In published GB Patent Application 2222247A there is disclosed a“distributed fiber optic sensor system”. In the disclosed system, pulsesof light which are shifted in relation to one another are transmittedalong a fiber. A pulse of light having a first frequency is scattered orreflected from a first location along the optical fiber and combined,after guidance back to a detecting element, with light scattered by thesecond pulse from a second location along the optical fiber. In additionto the fact that this disclosure states that it involves the analysis ofscattering from different sections of a fiber, the publication disclosesonly a single difference between the frequencies of the first and secondpulses.

Although these techniques seem useful, there are several limitations inthe disclosed systems. The most crucial limitation is reliability indetecting a threat, since missed detection would cause concern in manyapplications. The methods described above rely on a statistically randomvariable. Hence, at a given time, at any given position along the fiberunder test, the signal recovered from the coherent addition ofscattering from within the pulse has a finite probability of being closeto zero. In a real world application this would leave this part of thefiber unprotected. Due to the natural slow drift of the environmentalvariables, this “faded” fiber section would eventually drift back to asituation where it would again return a signal; however the “black outperiod” is still a major concern. Another concern is that the signalreturned from such a sensor is extremely non-linear and it may bedifficult to identify an acoustic disturbance since the acousticsignature is distorted by the generation of harmonics associated withthis non-linear response.

Accordingly, there is a need for an improved optical fiber, acousticdetection technique.

BRIEF SUMMARY OF THE INVENTION

The present invention involves an improved electromagnetic-wave,time-domain-reflectometry technique that may be used, in one specificembodiment, to detect acoustic waves in the vicinity of buried opticalfibers. In a specific application of the invention, such acoustic wavesmay foretell impending damage to the buried optical fiber, such as byimproper excavation. The technique, in one embodiment, involveslaunching into a medium, a plurality of groups of pulse-modulatedelectromagnetic-waves. The frequency separation between theelectromagnetic waves in two pulses within a first group is differentfrom the frequency separation between the electromagnetic waves in twopulses within a second group. A beat signal between the pulses of lightthat are scattered by the medium is then detected, and, in oneembodiment, that signal may be used to determine a characteristic of theenvironment in which the medium is located. For example, if the mediumis a buried optical fiber into which optical pulses have been launchedin accordance with the invention, the presence of acoustic waves withinthe region of the buried fiber can be detected.

These and other advantages of the invention will be apparent to those ofordinary skill in the art by reference to the following detaileddescription and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic embodiment of a basic ODTR architecture.

FIG. 2 is a representation of the backscattered intensity in a basicODTR implementation.

FIG. 3 is a schematic representation of coherent ODTR.

FIG. 4 is a specific embodiment of an aspect of the current invention.

FIG. 5 is a high level block diagram of a computer that may be used inimplementing an embodiment of the invention.

DETAILED DESCRIPTION Overview

The present invention involves an improved opticaltime-domain-reflectometry technique that may be used to detect acousticwaves in the vicinity of buried optical fibers. The invention will firstbe described first at a high level in terms of a specific embodiment.Thereafter the invention will be described with greater specificity.

The inventive technique, in one embodiment, involves launching opticalpulses into a buried optical fiber and detecting the signalbackscattered by the fiber. The optical frequency of one pulse within apair of pulses differs slightly from the optical frequency of the otherpulse within the pair of pulses. This frequency difference (orseparation) itself varies from one pair of pulses to the next. Thisvariation in frequency difference results in a detected backscatteredsignal having a phase that is modulated by an acoustic signal in thevicinity of the fiber, allowing decoding of the disturbance withimproved signal to noise ratio, reduced fading and a linear output. (Thepulses may be considered as individual pulses, pairs of pulses or groupsof pulses. The term “launching” includes introducing the pulse into thefiber or transmitting the pulse in the fiber. The term “optical” as usedherein may refer to the region of the electromagnetic spectrum that isvisible, generally considered to be between approximately 380 nm. and760 nm. However, depending on the application, the term “optical” asused herein can extend into what is sometimes referred to as theinfrared and ultraviolet range of the spectrum, for example from 200 nm.to 5,000 nm, or 10 nm. to 100,000 nm. In any event, the term “optical”will apply to any frequency which is used to transmit data or voice inor through optical fibers. While the discussion is in terms of anoptical fiber, in alternative embodiments pulses outside the opticalspectrum may be launched into any appropriate medium that will transmitthe pulses.)

In a specific embodiment where the pulses are 20 meters wide, thefrequency difference is on the order of 5 MHz and varies by about 5 MHzfrom one pulse pair to the next. In alternative embodiments thefrequency difference can range from approximately 1 MHz. toapproximately 5 MHz. For these parameters, each pulse pair results inindependent scattering, yielding improved signal to noise ratio. Theseparameters also result in a relative phase shift of the interferencebetween pulses within each pulse pair of the order of Pi, yieldingreduced fading if multiple pulse pairs are used. (Note that it ispossible to detect the scattering of each pulse pair from the samesection of fiber to within the spatial width of the pulse.)

Both the amplitude and the phase of the beat signal are affected byacoustic disturbances that may be present in the vicinity of the fiber.In one embodiment, the amplitude and phase is extracted from thescattered signal using known complex demodulation techniques.Demodulation is performed at the known frequency difference betweenpulses in a pair. Such demodulation may be performed by multiplying thereflected signal at the difference frequency by the sine and cosine ofthe difference frequency. This results in both sinusoidal components anda DC component. For a specific embodiment, a low pass filter withcut-off frequency in the vicinity of 5 MHz isolates the DC component(Note that if this low pass filter is too narrow it will blur thespatial resolution of the result and if it is too broad it will includethe results from the next pair of pulses that may be separated by about10 MHz. yielding a result that is not independent.) The DC componentreflects the phase and amplitude of the scattered signal. Rectangular topolar conversion enables the independent determination of the phase andamplitude. The phase will drift relative to the local demodulatingsignal because of small, slow-varying, changes in environmentalparameters. Accordingly, to remove this drift, a high pass filter isused on the phase signal after the rectangular to polar conversion. Inthis way only the acoustic signal is observed. Detection techniques thatonly measure the amplitude suffer from low signal to noise ratio, fadingand nonlinearity. The present invention, by using pulse pairs withappropriate frequency spacing and by analyzing the phase of thescattered beat signal, results in improved signal to noise ratio,reduced fading and an output that is linear over a larger dynamic range.

The discussion to this point has focused on a single pulse pair that istransmitted through the fiber. If, for this single pulse pair, we wantto look at every 20 meters of fiber, we sample accordingly in time,knowing the time of launch. In analyzing the results, N analysis “bins”may be used, one bin for each 20 meter section of fiber. When multiplepulse pairs are used at some pulse repetition rate then for each bindata will arrive at the pulse repetition rate. A filter is applied tolimit the signal to the acoustic band of interest—usually between 1 Hzand 200 Hz for acoustic coupling through the ground. A low pulserepetition frequency limits the maximum acoustic frequency that can bedetected without aliasing. A given frequency difference can not bereused in a second pulse pair until we have observed all of thebackscattering from that frequency difference. Accordingly, we can notreuse a given frequency difference until the round trip time within thecable passes. A 2.5 kHz pulse repetition rate is compatible with a cablelength of about 25 miles.

There is still an amplitude variation in the observed signal and if theamplitude gets too low the signal to noise ratio is poor. In thosecircumstances, the low amplitude result may be disregarded or given lowweight. Additionally, a phase unwrap algorithm may be used to obtaingreater dynamic range. Because of the discontinuity in arctan as thephase exceeds the range −Pi to +Pi, it is advantageous to add theresults at the discontinuity to remove that artifact. If there is noacoustic disturbance there is no change in the phase and amplitude. Ifthere is an acoustic disturbance, it results in very small local changesin fiber length and a linear change in the phase reflecting linearstrain in the fiber.

The duty cycle may be improved by launching pulse pairs with differentfrequency deltas for the various pulse pairs. In this way multiple pairsmay be propagated in the fiber at one time and their signals can bedifferentiated by observing the appropriate deltas. The analysis for asecond pulse pair is the same as that described above for the firstpulse pair except that the demodulating frequency is the new delta.After the high pass filtered phase result is obtained, it is combinedwith the appropriate bin from the previous pulse—using a time shiftreflecting the time difference between the first and second pulse pair.The resultant acoustic signals will add coherently—that is, if theacoustic signal is varying, the detected variation between the first andsecond pulse pairs will be in phase and add constructively resulting inimproved signal to noise ratio. Additionally, if one of the results forthe first pulse pair is faded or has low amplitude, the results for thesecond pulse pair is highly unlikely to show similar effects because ofthe pi shift in the deltas.

Specific Details

With the above overview as an introduction, the invention will now bediscussed in greater detail. As indicated above, the inventive techniqueenables a more robust and sensitive distributed acoustic sensing systemwith an increased duty cycle measurement of the disturbance and a linearresponse over a much greater dynamic range. The invention may be morereadily appreciated by those of skill in the art by considering theintensity of scattered light received from the coherent excitation of asingle mode fiber as the response of a traveling, randomly phase biasedinterferometer with the phase bias position of the interferometermodulated by any external change in the environmental variables. Thebias position of this interferometer is also a function of the opticalexcitation wavelength or frequency. For example, this virtualinterferometer has a differential length of the order of pulse length.Therefore as the optical wavelength is varied, the interferometer signalmoves through fringes—maximum constructive interference resulting whenan integer number of optical wavelengths exist in the interferometer andincreasingly deconstructive interference resulting until a minimumsignal is observed when the two interfering waves are out of phase, i.e.the signal is “faded”.

Given typical pulse lengths of the order of 20 m, the required change inoptical wavelength to go from a maximum to a minimum would be 6×10⁻¹⁴ m,or expressed in terms of optical frequency, 5 MHz, assuming an opticalwavelength of 1550 nm. Expressed in terms of the pulse duration, T, thismeans that the shift required in the spectral content of the lightcomprising the pulse has to be greater than 0.5/T. In the simplest form,two pulses are generated with different optical frequencies, F1 and F2,such that the difference between them is greater than the spectral shiftpreviously discussed (5 MHz). The pulses are arranged in time such thatthe second immediately follows the first. (Note, however, that the twopulses may be separated, contiguous or overlapping in whole or in part.)

Light at each optical frequency component is scattered as it propagatesalong the fiber. A small amount of this scattered light is then capturedby the fiber and guided back toward the launch where it is detected. Onthe detector, a beat signal is generated at a frequency equal to thedifference between the two optical components, i.e. (F2−F1). Theamplitude and phase of this carrier frequency is then measured as afunction of the time after the pulse was launched. Any disturbanceacting on the fiber will then effect both the phase and amplitude ofthis carrier, the range to the disturbance being determined in the samemanner as with a standard OTDR.

This processing method also allows two improvements. Since thedisturbance signal is now encoded and carried by the differencefrequency more than one such dual-frequency pair can be simultaneouslylaunched, and the disturbance signals from each independently extracted.The ability to independently extract the disturbance signal is due tothe fact that the carrier frequencies in each case are different thanpreviously launched and that the difference frequencies for each set arecarefully chosen. By also changing the absolute frequency as well as thedifference frequency of each pulse pair, in accordance with thisinvention, we can also significantly reduce the risk of signal fading aseach pulse set and each frequency component in the pulse set will bebiased at a different phase bias position within the virtualinterferometer.

The second advantage is the linearity of this encoding method. Since thedisturbance is now encoded as the phase of a backscattered signal, thesystem will return a linear response to strains within an opticalwavelength. In a somewhat more complicated implementation, the twopulses may be at least partly superimposed to generate a single pulsewith spectral components with a bandwidth of >0.5/T and launched intothe fiber. This has the advantage of increasing the spatial resolutionfor a given pulse energy.

FIG. 4 is a specific embodiment of an optical architecture of thecurrent invention. For simplicity in understanding FIG. 4, considerinitially only one wavelength, i.e. source 1 at 401. Light from thissource is multiplexed into a common fiber with light from source 2 viaDWDM 1 at 402. This light (both wavelengths) is then switched togenerate 2 pulses separated by a fraction of the pulse duration, by thepulse AOM at 403.

At this point may be helpful to describe the function of an AOM. An AOMcan switch the incoming light on and off (0^(th) to 1^(st) Order) withhigh extinction ratio, however in doing so it also shifts the opticalfrequency of the light by the same amount as the RF drive frequency usedto switch the device. In this embodiment we are using an 80 MHz AOM witha 20 MHz 3 dB Bandwidth. The two pulses generated by this first PulseAOM are not of the same frequency but are themselves offset relative toeach other by (F2−F1), where F1 and F2 are the RF frequencies used toswitch the AOM for the first and second pulses consecutively. Note that(F2−F1) is preferably >5 MHz for a 20 m pulse. (We have chosen 20 m and10 MHz for this first pulse pair.) As shown in FIG. 4, this generatestwo optical pulses of different optical frequency entering port 1 of thefirst 3 dB coupler, 404.

For simplicity, we will ignore the function of the loop—to which we willreturn later—and describe what happens to this first pulse pair. Fromport 1 of the first 3 dB coupler, 404) the pulse pair propagates througha fiber isolator, 405, before being incident on a second 3 dB coupler406. Half of the energy stored in each pulse is therefore routed evenlyinto the two output arms of this device. The light in one arm is thendelayed by an amount determined by the length of the fiber reel, Delay2, 407. If the delay between the pulses is set equal to Delay 2 thenwhen the pulse pair is recombined with its delayed copy via 3 dB3 F1will be entirely superimposed on F2. The actual delay between pulses iscontrolled electronically to allow the final launch pulses to beseparate, or to control the degree to which they are superimposed. Aportion of the light is also monitored via the remaining output port bythe “Diagnostics detector”, 408.

The delay and recombination generates a pulse pattern consisting of adelayed F1 pulse, a pulse consisting of the delayed F2 and the originalF1 and also a third pulse, the original F2. As will be highlightedlater, we will select the portion of the pulse train containing both F1and F2 components. The other pulses are later discarded by the ASE AOM,409.

The pulse train is amplified by the dual stage optical amplifier, 410consisting of, Pump 2, PWDM 2, EDFS 2, Isolator 3, EDFS 3, PWDM 3, Pump3 and Isolator 4. The gain of the amplifier is controlled by an inlinefiber attenuator, 411. The amplified pulse train then enters thewavelength delay stage via port 1 of CIRC 1, at 412, and exits port 2.Depending on its wavelength the pulse train is then reflected by thecorresponding narrowband grating at 413. If the pulse train originatedfrom the lower wavelength source then it is not delayed. However, thehigher wavelength pulse train is reflected after Delay 3, introducing adelay equal to twice this delay length. The dual wavelength nature ofthis embodiment improves the reliability of the system by providingwavelength diversity and hence an extra level of fading reduction. Thereflected light then travels back to port 2 of the circulator, 412, andexits via port 3. It is at this point that the ASE AOM allows only therequired portion of the pulse train containing the F1 and F2 componentsto pass. This temporally filtered and amplified version of the originalpulse train is then incident on Circ 2, 414. It enters via port 1 andexits via port 2 into the fiber under test, 415.

As the pulse containing F1 and F2 components propagates along the fiber,a small portion of the light guided by the fiber will be scattered, aswith any OTDR, captured by the fiber and returned in the oppositedirection along the fiber. The illumination is coherent and thus thelight at F1 and F2 both scatter an amount of light which is related to astatistically random variable based on the fiber characteristics at anygiven time. Any change in the environment of the fiber such as strain ortemperature results in a change of the amount of light scattered at F1and F2. The scattered light is then routed via port 3 of Circ 2, 414, tothe detection system.

The returning signal is amplified by the pre-detect amplifier,consisting of Pump 4, PWDM 4, and EDFS 4 all shown at 416. A fourth AOM,417, is included to block any high intensity light from strongreflectors saturating the detection system. As with the launch, awavelength-dependent delay arrangement is used at 418. However thewavelengths that are delayed or simply reflected are reversed whencompared to the launch section, 407. This removes the offset between thesignals generated by Source 1 and Source 2, hence simplifying theprocessing required. An attenuator, 419, is then included before thelight is routed to one of two detectors by DWDM 2, 420, depending on itswavelength. In this way detector 1 observes the signal generated bysource 1 and detector 2 that by source 2. At the detector, the twooptical frequencies at F1 and F2 interfere and generate a differencefrequency, i.e. (F2−F1).

The operation of the loop, 421, at the launch end of the optics is togenerate additional pulse pairs derived from the original set, but alsoto increase the difference frequency between them on successive passes.Consider the first pulse pair, one at F1 and the other at F2 enteringthe loop via port 4 of 3 dB 1, 404. They first enter delay 1 to allowtime for the entire pulse pair to enter the loop. These pulses are thenamplified to account for any losses within the loop, the gain beingcontrolled by an attenuator, which is then isolated to preventreflections creating gain instability in the amplifier. As the pulsepair propagates around the loop the first pulse enters the cavity AOMwhich allows it to pass and adds an additional F1 frequency shiftresulting in a pulse shifted by 2F1 in frequency. A short time later aspulse F2 enters the AOM it is switched to run at F2 and hence generatesa pulse shifted by 2F2 in frequency. The second pulse pair however hasdouble the frequency difference of the first, i.e. 2(F2−F1) and also itsabsolute frequency has also been increased by (F1+F2)/2. This pulse pairis then returned to the system, as was the first, via port 3 of 3 dB1,404, however it is also reentered back into the loop. Additional pulsepairs can therefore be constructed, separated in time by delay 1—i.e.with each transit of the loop the pulse pair is shifted by an additional(F1+F2)/2 in absolute frequency and (F2−F1) in difference frequency

A specific embodiment of the signal processing involves an analysis ofthe beat frequency. The electronic output signal from the opticaldetector in the above system comprises a number of (one or more)carriers with frequencies f, 2f through Nf where f is of the order of 10MHz. Each of these carries acoustic disturbance information as amplitudeand phase modulation. Moreover, with no acoustic disturbance present,there is a random amplitude modulation due to the optical “fading”mechanism described above. There are limitations associated with the useof direct intensity or carrier amplitude to represent acousticdisturbances. For example, the acoustic sensitivity with respect tolongitudinal distance corresponds to the absolute intensity of thebackscattered signal from each location, which is random. This meansthat it is not possible to make quantitative measurements. The sign aswell as the magnitude of the sensitivity at each location is random.This is directly related to the random bias point of the virtualinterferometer created by the optical pulse. The response of the virtualinterferometer is non-linear for disturbances large enough to cause aneffective path length change that is a significant fraction of awavelength at the optical frequency.

The inventive method and system also carries the acoustic disturbancesignal as phase modulation of the carriers. If phase information isextracted independent of amplitude fluctuations the above limitationsare overcome. There are additional advantages of the present invention.For example, phase demodulation will return a linear response over afull optical wavelength. Additionally, a phase unwrap algorithm may beapplied which will be effective for the large, low frequencydisturbances expected in the application. This will extend the lineardynamic range over an arbitrary number of optical wavelengths. Thedemodulated acoustic signals from each RF carrier corresponding to onelocation may be combined to increase SNR and to remove susceptibility tothe effects of fading. Details of the preferred implementation of themethod are provided here, although any substantially equivalentimplementation maybe used.

The signal from the optical detector is low-pass filtered with anappropriate anti-alias filter and sampled by an ADC at an appropriaterate. The resolution and performance of the ADC is chosen such that itdoes not contribute significantly to system noise. By way of exampleonly, the method will be described using the following assumptions:there are 3 RF carriers present: 10 MHz, 20 MHz & 30 MHz; the ADC samplerate is 100 MHz; the refractive index of the optical fiber is 1.46;required spatial resolution is 20 meters; and, the total length of thefiber is 20 km. The sampled digital data is passed to a 3 channelcomplex demodulator that uses 3 numerically controlled sine & cosinelocal oscillators (NCOs), each set to one of the nominal carrierfrequencies to provide independent In-phase (I) and quadrature (Q)signals from each of the carriers. It is preferred that the NCOs havetheir frequency derived from the same reference as the optical signalmodulators and are arranged to have exactly the same frequency as the RFcarriers. This simplifies signal processing because, for steady stateand no disturbance, the base-band signals will be at zero frequency (butarbitrary phase).

The I & Q signal pairs (hereafter: complex signals) are low passfiltered using finite impulse response filters to define the spatialresolution of the system. The temporal response of these filters definesthe level of discrimination of the system's response to one spatiallocation versus its neighbors. Using the assumptions above, the inputsamples to these filters correspond to a spatial displacement of:c/(2*100 MHz*1.46)=1.027 meters.

The frequency response should preferably provide sufficient anti-aliasfiltering of broad-band noise from the optical detector for thesubsequent decimation such that this does not contribute significantlyto overall system noise. The output of these filters is decimated toprovide the required spatial resolution. In the example, decimation by20 would provide approx the required 20 m resolution (actually: 20*1.027=20.54 m). Each of the complex signals is time divisionde-multiplexed into a number of separate signal processing channels, onefor each spatial resolution point. The number of channels required foreach complex signal in the example is: 20 km/20 m=1000.

Each of these de-multiplexed complex signals now corresponds to aparticular, fixed location along the length of the fiber. Any strain onthe fiber due to an acoustic signal will modulate the phase (andamplitude) of the complex signal. Each of these channels provideslow-pass filtering to include the spectrum of the acoustic signal ofinterest while removing unwanted acoustic signals (e.g., from distantroads) and reducing noise. The complex (independent I & Q) signals arethen converted to phase and amplitude using: phase=arctan (Q/I);amplitude=√(I²+Q²). The phase signal is high-pass filtered with acut-off frequency appropriate to the lowest acoustic signal of interest.This will remove the arbitrary DC phase component and reduce the effectof low frequency system noise sources such as temperature fluctuationsof the fiber.

The filtered phase signal now relates directly to the optical phasechange caused by the acoustic disturbance at the remote location. Atthis point it is possible to combine the phase signals from each of theindependent RF carriers corresponding to the same location. If allcarriers at a given location have the same amplitude, the effect ofcombining them will be to coherently add the acoustic signal but toincoherently add any system noise. In this case a √N increase in signalto noise ratio will be achieved, where N is the number of independent RFcarriers. This corresponds to 4.8 dB SNR improvement in the example.

Another benefit of the combination of acoustic signals from the RFcarriers is the very effective reduction in susceptibility to fading itaffords. For example, if the probability that a given location will haveits sensitivity reduced by at least 20 dB due to fading is 1%, then theprobability that all three independent RF carriers will be faded by thisamount for the same location is 1% cubed or 0.0001% or 1 ppm. In orderto avoid the potential SNR degradation that would be caused by combiningthe phase signal for an RF carrier that has low SNR (e.g., due tofading), amplitude qualification criteria are used before combiningphase signals. In its simplest form this may be a simple threshold setat a fraction of the amplitude of the greatest of the 3 independentsignals. RF carriers with amplitude greater than this threshold havetheir phase signal combined; those that don't are ignored. An optimalimplementation uses a weighted contribution from each based on carrierSNR.

As discussed, the dynamic range of the system may be greatly increasedby, applying a phase unwrap algorithm. In its simplest form, this may beachieved by adding or subtracting Pi radians if consecutive phaseresults are different by greater than Pi/2. This will be effective evenin poor SNR situations as long as the amplitude-frequency product of theacoustic disturbance is insufficient to cause greater than Pi/2 phaseshift between optical interrogation pulses. A more robust approach is touse a form of signal predictor (eg: a Kalman filter) and to select froma number of integer cycles (multiples of 2Pi radians) in eitherdirection for the least error versus the predictor. Extending thedynamic range of the system upwards in this way allows a much higherthreshold to be set which removes false alarm problems at locationswhere the expected environmental noise is high (eg: fence borneapplications). Additionally, because the system remains linear even inthe presence of high environmental disturbances (e.g., wind noise)—wheredirect intensity based systems give a clipped, highly non-linearresponse—it is possible to take advantage of further processing forsignature recognition or geo-location even in high noise applicationssuch as surface, fence or overhead deployment of the fiber

The foregoing Detailed Description is to be understood as being in everyrespect illustrative and exemplary, but not restrictive, and the scopeof the invention disclosed herein is not to be determined from theDetailed Description, but rather from the claims as interpretedaccording to the full breadth permitted by the patent laws. It is to beunderstood that the embodiments shown and described herein are onlyillustrative of the principles of the present invention and that variousmodifications may be implemented by those skilled in the art withoutdeparting from the scope and spirit of the invention. Those skilled inthe art could implement various other feature combinations withoutdeparting from the scope and spirit of the invention. For example, andwithout limitation, much of the discussion has been in terms of opticalfrequencies launched into a medium comprising and optical fiber.However, those skilled in the art will understand that the invention canbe used in any area of the electromagnetic spectrum for appropriatemedia.

Additionally, the invention can be implemented as a set of instructionson a computer readable medium. The computer, in such an embodiment, willbe similar to those that are well known in the art, and may beimplemented, for example, using a well known computer processors, memoryunits, storage devices, computer software, and other components. A highlevel block diagram of such a computer is shown in FIG. 5. Computer 500by executing computer program instructions which define such operation.The computer program instructions may be stored in a storage device 504(e.g., magnetic disk) and loaded into memory 505 when execution of thecomputer program instructions is desired. Thus, computer programinstructions may be stored in memory 505 and/or storage 504 and theapplication will be controlled by processor 503 executing the computerprogram instructions. Computer 500 also includes one or more networkinterfaces 501 for communicating with other devices via a network.Computer 500 also includes input/output 502 which represents deviceswhich allow for user interaction with the computer 500 (e.g., display,keyboard, mouse, speakers, buttons, etc.). One skilled in the art willrecognize that an implementation of an actual computer will containother components as well, and that FIG. 5 is a high level representationof some of the components of such a computer for illustrative purposes.

1. A method comprising: launching into a medium a at least four pulsemodulated electromagnetic waves, each of said pulse-modulatedelectromagnetic-waves having an associated electromagnetic-wavefrequency, the frequency separation between the first and second of saidwaves being different from the frequency separation between the thirdand fourth of said waves; and detecting a characteristic of a beatsignal between a given pair of electromagnetic waves that are scatteredby the medium. 2-26. (canceled)